Self-regulating expansion type control valve

ABSTRACT

The lifting of a drop hammer is controlled by a spool-type valve admitting compressed air into the main cylinder of the hammer. In order for compressed air to be admitted the valve spool is forced from its normal position exhausting the cylinder into a position supplying compressed air to the main cylinder by admitting control air into a region in which its pressure moves the spool against opposition by a spring. After compressed air has been admitted to the main cylinder, at a preselected control position, the control air is exhausted. A slide in the valve is then moved toward the spool and into a stop by air pressure from the cylinder. The spring returns the spool part way toward normal position until it contacts the slide, in which position the cylinder is closed off from the supply of compressed air but is not opened to exhaust. When the air pressure in the cylinder has been reduced by expansion to a predetermined level at which most of its energy is extracted the spool under urging of the spring will push the slide out of the way and the spool back to position to exhaust the cylinder.

United States Patent 72] Inventor Charles W. Frame Chambersburg, Pa.

[21 1 Appl. No. 799,277

[22] Filed Feb. 14, 1969 [45] Patented Mar. 2, 1971 [73] Assignee Chambersburg Engineering Company Chambersburg, Pa.

[54] SELF-REGULATING EXPANSION TYPE CONTROL 3,470,792 l0/l969 Darling Primary Examiner-Everette A. Powell, Jr. AttorneyHowson and Howson ABSTRACT: The lifting of a drop hammer is controlled by a spool-type valve admitting compressed air into the main cylinder of the hammerrln order for compressed air to be admitted the valve spool is forced from its normal position exhausting the cylinder into a position supplying compressed air to the main cylinder by admitting control air into a region in which its pressure moves the spool against opposition by a spring. After compressed air has been admitted to the main cylinder, at a preselected control position, the control air is exhausted. A slide in the valve is then moved toward the spool and into a stop by air pressure from the cylinder. The spring returns the spool part way toward normal position until it contacts the slide, in which position the cylinder is closed off from the supply of compressed air but is not opened to exhaust. When the air pressure in the cylinder has been reduced by expansion to a predetermined level at which most of its energy is extracted the spool under urging of the spring will push the slide out of the way and the spool back to position to exhaust the cylinder.

Patented March 2, 1971 3,566,747

I. Sheets-Sheet 1 mvzw'ron: CHARLES W. FRAME Patented March 2, 1971 L Sheets-Sheet 5 FIG.6.

FIGS.

SHORT STROKE STANDARD VALVE FIG] HANNE Ua.

CHANNEL 4.

SHORT STROKE-EXPANSION VALVE CHANNEL 5.

CHANNEL 4.

INVENTOR'. BY CHARLES W. FRAME J/WV ATTYS.

Patented March 2, 1971 3,566,747

C Sheets-She t FIGB.

FIGQ.

W INVENTOR'.

BY CHARLES W. FRA

ATTYS.

SELF-REGULATING EXPANSION TYPE CONTROL VALVE This invention relates to a self-regulating expansion type control valve for various work cylinders operated by a compressible fluid. It has particular application to air hammers and other impact devices, for example, of the type described in the U.S. Pat. Nos. 3,142,206 and 3,043,271.

In the prior art, devices of the general type which may be advantageously controlled by the system of the present invention, have been controlled eithermanually or automatically, by valves, which have merely alternately directed compressed air into the main cylinder and exhausted air from the cylinder. This has been a wasteful and expensive practice, since not all of the energy of the compressed air has been used. This energy has been wasted because air at pressure substantially above atmospheric pressure has simply been exhausted to the atmosphere.

In a conventional system air expands at least to some degree because of the inherent capability of the cylinder to enlarge as the piston moves more rapidly than air flow can replace it, or alternatively because means have been provided for cutting off the supply of air at a particular time. In either case, the degree to which air is allowed to expand is commonly controlled either to a fixed time by a timer or by the position of the piston. However, since the proper time or the proper piston position for maximum efficiently depends upon variable operating parameters, there can be no assurance that maximum efficiency will be obtained. Some of the variables which make it impossible to predict proper time or position, even in the same machine, are the required output of energy from the cylinder, the required stroke of the cylinder, and the weight that the cylinder must accelerate, and the inherent nature of material being forged.

The present invention enhances the efficiency of the control regardless of the variableness of the operating parameters. The self-regulating expansion valve of the present invention senses the cylinder pressure and, in effect, compares this pressure to a predetermined standard. When the difference between the cylinder pressure and atmospheric pressure approaches a preset minimum difference, the valve opens. In this way the intrinsic energy of the spilled or exhausted air is minimized. 4

More specifically, the present invention relates to a method of obtaining maximum energy at minimum cost in terms of fluid consumption from the expansion of compressed fluid used to do work moving a piston in a cylinder. The method consists of introducing compressed fluid at a predetermined pressure into the cylinder, shutting off the cylinder from the compressed fluid supply at a predetermined pressure and allowing the fluid in the cylinder to move the piston by expansion. Force regulated by fluid pressure is provided in the cylinder to oppose opening the cylinder to exhaust and exhaust is permitted to occur only when pressure in the cylinder drops below a preselected minimum.

The invention further relates to a machine having at least a piston in a cylinder, said piston being moved to do work by the introduction of a compressed fluid into the cylinder on one side of the piston. The machine particularly employs a selfregulating expansion valve means to alternatively connect the cylinder to a compressed fluid supply or to exhaust, or to close the cylinder so that the cylinder is cut off from both supply and exhaust. The valve means includes relatively movable valve members including ports connectable to exhaust, to the air supply and to the cylinder. Resilient means urges the member into a position in which the exhaust port is connected to the cylinder port and means selectively opposing the resilient means is provided to move the members into a position connecting the air supply to the cylinder. Finally, means responsive to the pressure of the expanding air in the cylinder holds the members in the intermediate position when the opposition of the means selectively opposing the resilient means is released until the pressure is reduced to a predetermined level.

For a better understanding of the present invention reference is made to the accompanying drawings in which:

FIG. 1 is a front elevational view of a pneumatically actuated gravity drop hammer employing the present invention;

FIG. 2 is a side elevational view of a portion of the drop hammer of FIG. 1;

FIG. 3 is an enlarged sectional view of the throttle valve;

FIG. 4 is a sectional view of similarly enlarged scale showing the main control valve of the drop hammer in normal position, wherein the main cylinder is exhausted and the hammer is at the top of its stroke as shown in FIG. 1;

FIG. 5 is a sectional view is similar to FIG. 4, but showing the valve in a position into which it is moved to initiate raising the drop hammer;

FIG. 6 is a sectional view similar to FIGS. 4 and 5, showing the valve in the course of raising of the hammer;

FIG. 7 is a comparison chart showing plots of ram displacement against time and plots of main cylinder pressure against time;

FIG. 8 is a schematic diagram of a single acting drop hammer of the type shown in FIGS. 1-6; and

FIG. 9 is a similar schematic diagram showing a double acting drop hammer of the same general type employing the present invention.

Referring to FIG. 1, as mentioned above, the gravity drop hammer shown in FIG. 1 is of the type disclosed in the U5. Pat. 3,142,206 and 3,043,271 of Wilmer W. Hague and Glenn A. Householder and U.S. Pat. 2,604,07l to Paul A. Rickrode. Such devices characteristically have their hammer or ram raised by compressed air, or other suitable compressible compressed fluid. Their hammer drops when the compressed air is exhausted from the main cylinder and successive blows will occur unless the hammer is stopped. To prevent successive blows from being struck clamping means is used to grasp and hold the hammer at the top of its stroke. The clamping means is then released whenever the hammer is intended to strike a further blow.

Referring particularly to FIG. 1, the drop hammer is formed by bolting together a base 1, which may be partially buried in foundation concrete, relatively spaced apart side frames 2 and a head or upper housing 3. Upper housing 3 mounts a vertically arranged main cylinder 4 containing a piston 5. Piston 5 is connected to the upper end of piston rod 6, at the lower end of which is connected ram 7. Ram 7 is mounted for vertical reciprocatory movement guided by track means 8 on the side frames 2. A replaceable ram die 70 is affixed to the ram in conventional manner by means of a tapered wedge. In the bottom-most position the ram die 70 impacts against a replaceable anvil die 9a which is similarly affixed to the anvil cap 9.

In accordance with the practice usually employed in compressed fluid operated drop hammers, the ram 7 and die 70 are adapted to drop freely by gravity from the elevated position shown in FIG. 1 downwardly into impact engagement with the anvil die 9a. Fluid under pressure is then admitted to the cylinder 4 below the piston 5 to return, or raise the ram 7 to elevated position after each drop. Since the compressible fluid providing pressure utilized to actuate the ram-lift piston is commonly air, it will be described as air herein. The compressed air is under the control of a main valve 10, which will be described in substantially greater detail below.

Compressed air 5 supplied through the main valve 10 to the cylinder 4 from a suitable compressed air supply source (not shown) to pressure inlet 12 and through a throttle valve II. The throttle valve 11 acts to selectively control the rate of flow to the main valve 10 and, when permitted by the main valve, to the cylinder 4. The throttle valve is controlled through mechanical push-pull control cable 13 to a manual throttle adjustment mechanism 14 which allows the throttling effect to be selected and held at any desired level to control the rate of lift of the ram. Exhaust to the atmosphere from the cylinder 4 through a vent 15, is also controlled by the main valve 10. a

To actuate the main valve 111 into position to raise the ram, compressed air is employed at a control pressure, which may be the same as the pressure supplied to the main cylinder from the same compressed air source. This pressure is provided to the control region of the main valve 111, by suitable control valve means 47, which is shown schematically at FIG. 4. The control valve is a 2-position, 3-way, normally closed, air operated, solenoid controlled valve.

Pressure preferably from the same source, is also supplied to a control valve for actuator 16 to actuate mechanical clamping mechanism 17. Actuator 16 is operated by mechanical push-pull control cable 18 controlled by treadle 19. Treadle 19, when depressed against the action of spring 20, causes actuator 16 to release the clamping mechanism 17, which acts to hold piston rod 6 and ram 7 from downward movement.

When the ram is allowed to fall, its position is sensed by stroke control sensing apparatus 22. The control housing 26 contains program control circuitry which selectively makes either switch 23 or 24 effective depending upon whether a short or long stroke has been programmed on each particular drop of the hammer. Whichever switch is rendered effective, in turn, starts a timer which determines by timing the length of the stroke from the point of actuation of the effective switch as the ram falls. Switch 25 is a safety switch provided to shut down the hammer closing the main valve in the event the ram overtravels. Switch 23, if energized in a particular stroke, will produce a short stroke, whereas if switch 24 is instead energized the stroke will be long. Each switch is effective to energize a pair of electronic timers. One timer determines when the main valve shall open by energizing pilot valve 47 to admit air into the main cylinder beneath the piston to raise the ram. The other timer determines when the main valve closes by deenergizing pilot valve 47 to block the flow of air into the cylinder 4. In essence one timer programs the ram pickup position and the other programs the ram stroke.

As can be seen in FIG. 3 the throttle valve is simply a tubular structure 27 arranged transverse to the air pressure supply duct 28 and having opposed openings through its sidewalls which by rotational adjustment tend to align or become partially misaligned with the inlet duct. The tubular valve 27 is positioned and controlled by a crank arm 29 operated through the control cable 13 and handle 14 as previously described in connection with FIGS. 1 and 2. The throttle valve is located in the pressure inlet duct 28 before the air pressure supply chamber 30 of the main valve 10.

Main valve is shown in FIGS. 4, 5 and 6. This valve consists of a cast housing 31 providing a stationary valve member through which extend ducts and a bore generally normal to these ducts at the ports where they communicate with the bore. Inlet duct 30 broadens into an annular chamber 30a around the bore. An exhaust duct 32 leading to exhaust outlet (FIG. 2) begins at the bottom of the bore in annular chamber 35 A duck 34 connects the cylinder 4 and the annular chamber 35 surrounding the bore. The bore receives a tubular sleeve 36 which provides an inner wall for each of the annular chambers and provides a bearing for the movable valve member slidable spool 37. Spool 37 has two axially separated circumferential flange areas 37a and 37b which have peripheral contact with the sleeve 36 and are provided with appropriate sealing means to prevent flow between either of them and sleeve 36. The spool 37 also has an axially oriented cup which receives one end of helical coil compression spring 38, the other end of which is received within cuplike retaining structure 39 to urge the spool into the position shown in FIG. 4, which is the normal position. The spring 38 urges the spool toward the end of the valve structure and into slide-stop 41. The valve bore is extended to provide room for slide-stop 41. A sleeve 40 in the extended bore of somewhat larger internal diameter than the internal diameter of sleeve 36 provides the bearing surface permitting sliding of slide-stop 41. Cup-shaped slide-stop 41 is seen in FIG. 4 in a position resting against an upper stop 42, preferably of resilient material which, in turn, lies against the solid stop member 43. Stop member 43 is mechanically part of a fixed valve structure casting 31. A portion 44 of stop member 43 is designed to occupy most of the volume of the cup of slide-stop 41, but is spaced from the internal surface of it.

It will be observed that both spool flange 37b and the bottom of sidewalls of the cup slide-stop 41 have sealing members and a space is provided around the outside of the upper part of the spool 37 by virtue of the larger diameter of sleeve 40. This space is connected at the bottom of sleeve 40 by ports 411a to a duct 45 through which pressurized control air may be introduced. A spacer 370 on the end of spool 37 separates the spool from the bottom of slide-stop 41 so that air from duct 45 can flow between these two members and separate them.

A duct 46 also passes through the stationary valve member 31 and through ports 40b in sleeve 40 to a point always above the seal 41a, even when the slide-stop 41 is in its uppermost position shown in FIG. 4. The outside cup sidewalls of slidestop 41 are recessed 410 to permit communication between port 40b in sleeve 40 and port 41b through the cup sidewall. In this way duct 46 connects the duct 34, in communication with the cylinder 4, to the interior of the cup slide-stop in all of its positions.

It will be observed that in the position shown in FIG. 4, with the valve spool in its uppermost position, there is direct communication between the exhaust duct 32 and the duct 34 to cylinder 4. In this position, should the ram be unclamped by depressing the treadle 19 to release the clamp 21, the ram die 7a would fall by gravity to impact the anvil cap die 9a. In so doing, it would pass and actuate switches 23 and 24, which using conventional mechanism opens a valve 47 directing air pressure through duct 45 and passages 40a into the space between the spool valve 37 and the slide-stop 41. This pressure will tend to drive the spool 37 downwardly into the position shown in FIG. 5, wherein the bottom of the spool valve is stopped against top edge of the spring support 39. In this position, the exhaust duct 32 is no longer in communication with the channel 34 leading to the cylinder 4, but is cut off from channel 34. Instead, channel 34 is in communication through the ports 36b and 36 a, respectively, with the air pressure supply as it has reached annular region 30a. Air pressure thus received in the cylinder 4 will cause the ram to be pulled upward as its piston 5 is raised in cylinder 4. As the ram raises and in accordance with a preset program, the control valve 47 is closed to remove the pressure from the space between the spool 37 and the cup slide-stop 41, and, in fact, preferably to exhaust said area to atmosphere. When this happens the spool valve 37 is urged upwardly by spring 38. However, at the same time the slide-stop 41 is urged downwardly by the pressure in the cylinder 4 through its channel 34 and duct 46, which lead by ports 40b and 41b into the space within the cupshaped slide-stop 41. The pressure on the slide-stop urges the slidestop downwardly in direct opposition to the spool 37 and the pressure in the cylinder 4 is more than sufficient to overcome the spring 38. However, slide-stop 41 is of larger diameter than the internal diameter of the sleeve 36 and it therefore comes to rest atop the end 36c of sleeve 36 which constitutes a shoulder, or stop discontinuity, preventing further movement of the cup slide-stop 41. Ports 40a enable escape of air trapped between slide-stop 41 and spool 37 to duct 45. In this position, the position shown in FIG. 6,- the flange 37a cuts off ports 36a entirely from communication with either exhaust channel 32 or pressure supply 30a. Therefore, pressure within the cylinder 4 is maintained at the level it achieved when cut off from its air supply subject only to reduction as the volume is increased due to the rise of a piston 5 within the cylinder. As the cylinder volume increases, the pressure gradually declines, as can be seen in the diagrams marked D (channel 04) in FIG. 7.

As the pressure diminishes to a point where the spring pressure exceeds the air pressure, the spring will overcome the air pressure moving the slide-stop 41 and the spool 37 both upwardly until they reach the position shown in FIG. 4. This occurs as the ram approaches its selected height, depending upon the preset impact blow program. At the top of any stroke, whether long or short, the ram may be stopped by the clamp 21 through actuation of valve 16 by release of the foot treadle 19. If not stopped blows will continue to be repeated in accordance with the program selected until the clamp is engaged to stop the ram.

FIG. 7 compares performance of a ram in short stroke using a standard valve and an expansion valve of the present invention. Curves B and D plot pressure in the cylinder 4 against time. The cylinder pressures of curves B and D result in successive vertical ram positions shown in curves A and C, respectively, which are plotted on the same time scale. These plots are typical of test plots made under working conditions through many cycles and are consistent with the results of other tests. In these particular tests, the length of the ram stroke when the valve of the present invention was employed was 16%inches (curve C), whereas the stroke of the ram with a standard valve was only 14% inches (curve A). These plots were made using the same equipment comparing the results over many cycles.

Comparing curves B and D the plots show successive cycles in which pressure to the cylinder first increases sharply followed by a gradual rise in pressure in each case. The valve of the present invention remains in the position of FIG. 5 during this period. Thereafter, as pressure is discontinued, there is a drop-off in each case, but in the case of curve B the drop-off is more pronounced and the effect consequently terminates more rapidly. In curve D, by contrast the drop-off is more gradual and the effect is sustained longer due to retention of the valve in the position of FIG. 6 during this period as the air in the cylinder expands. This sustainedperiod of expansion carries over in sustaining the rise of the ram for a longer period, as can be seen by comparing curves C and A, and, therefore, the ram rises to a higher point with the expansion valve and methods of the present invention using the same amount of energy Superimposing curve B on curve D as shown in dashed lines result in defining an area between the curves in the expansion portion of the cycle. This area isproportional to the energy saving provided by the present invention.

A particular form of the present invention has been described. It will occur to those skilled in the art, that many modifications can be made.

One such modification is application of the system to a double-acting hammer in which the hammer is positively impelled both in lifting and driving toward the work. Such a hammer is shown schematically in FIG. 9. For the sake of convenience, in direct comparison, a schematic diagram of the single acting hammer already described has been shown in FIG. 8. As seen in FIG. 8, the piston 5 is raised in cylinder 4 in response to the special main control valve 10, already described. The rate is controlled to some extent by the throttle valve 11. The action of the control valve in initiated by the solenoid control pilot valve 47 which supplies pilot air to the control valve only when activated by the solenoid. The air supply, for the control valve and for the solenoid control pilot valve, may be derived from the same source as shown.

FIG. 9 shows schematically a double-acting hammer whose ram 7' is moved by feeding air into the cylinder 4 alternately on opposite sides of the piston 5 in order to drive the ram 7 through piston rod 6 sequentially upwardly and downwardly. In order to supply air to the opposite sides, a pair of similar control valves 10a and 10b are employed, each of which is operated by its own solenoid control pilot valves 47a and 47b. A single throttle valve 11' may be employed to feed the control valves 10a and 1012. It will be appreciated that the sequencing is controlled by a preset program controller, which determines among other things when solenoid control pilot valves 47a and 47b are operated. Those skilled in the art will understand the need for proper sequencing of the action of the valves 10a and 10b, which is taken into consideration in the programming of the operation of solenoid control pilot valves 47a and 47 b. In each case air expansion self-regulated by the main control valve takes place exactly as with the self-regulating control valve of the system described above, and there is a similar expansion as the hammer is driven in each direction All such modifications within the scope of the claims are intended to be within the scope and spirit of the present invention.

I claim:

1. In a machine having at least a piston in a cylinder, said piston being moved to do work by the introduction of a compressed fluid into the cylinder on one side of the piston; a selfregulating expansion valve capable of assuming three discrete positions to alternatively connect the cylinder to a compressed fluid supply or to exhaust the cylinder or to close the cylinder so that the cylinder is cut off from both supply and exhaust, comprising:

relatively movable valve members including ports connectable to exhaust, to the air supply and to the cylinder;

resilient means urging the member into a position in which the exhaust port is connected to the cylinder port;

means selectively opposing the resilient means to move the members into a position connecting the air supply to the cylinder; and

means responsive to the pressure of the expanding air in the cylinder to hold the members in an intermediate position in which the cylinder is closed when the opposition of the means selectively opposing the resilient means is released until the pressure is reduced to a predetermined level.

2. The machine of claim 1 in which the ports are in the stationary member of the valve.

3. The machine of claim 2 in which the movable member of the valve is linearly slidable.

4. The machine of claim 3 in which the resilient means is spring means, or other means providing a nearly constant force.

5. The machine of claim 4 in which the means selectively opposing the spring means includes a fluid pressure supply means which acts on the movable valve member to overcome spring pressure.

6. The machine of claim 5 in which the means responsive to the expanding air is a slide-stop which intercepts the slidable member in its return to exhaust position and holds it out of exhaust position and in a position in which the cylinder port is closed.

7. The machine of claim 6 in which the slide-stop provided with pressure from the cylinder through a duct to the side of the slide-stop opposite that contacting the slidable member.

8. The machine of claim 7 in which shoulder stop means are provided to limit the movement of the slide-stop to a position which will hold the slidable member in position to close the cylinder port.

9. A slide type valve system comprising:

a fixed member having ports leading respectively; to exhaust, to a cylinder, and to an air pressure source;

a slidable member having limited movement relative to the fixed member in opposite end positions of which the cylinder is connected to exhaust and to air pressure, respectively and in an intermediate position of which the cylinder port is closed to close off the cylinder;

resilient spring means between the slidable and fixed member urging the slidable member into the position connecting the cylinder to exhaust;

air pressure means selectively applicable to the slidable member in opposition to the spring means to move the slidable means to connect the cylinder to the source of air pressure; and

a slide stop in axial alignment with the slidable member and movable in response to air pressure in the cylinder into a position to oppose movement of the slidable member into exhaust position and closing the cylinder off from the other ports until the air pressure in the cylinder is reduced to a level which can be overcome by the spring means to return the slidable member to the position connecting the cylinder to exhaust.

10. The structure of claim 9 in which the slidable member is a spool urged by compression spring means between the spool and the fixed member into the position connecting the air pressure supply to the cylinder.

11. The structure of claim 10 in which the fixed member is provided with a duct leading from the cylinder to the other side of the slide-stop from that contacting the slidable member, and in which a shoulder for said slide-stop is provided to limit movement of the slide-stop to a position in which it holds the spool in its intermediate position wherein the cylinder port is closed, so that air pressure in the cylinder holds the slide open until the pressure of the air in the cylinder is reduced to the point where the spring overcomes the pressure.

12. The method of obtaining maximum energy from the expansion of compressed fluid used to do work moving a piston in a cylinder comprising:

introducing compressed fluid at a predetermined pressure into the cylinder;

shutting off the cylinder from the compressed fluid supply at a predetermined time and allowing the fluid in the cylinder to move the piston by expansion;

pressure in the cylinder providing a force regulated by fluid to oppose opening the cylinder to exhaust the cylinder; and

permitting exhaust to occur only when pressure in the cylinder drops below a preselected minimum.

13. The method of claim 12 in which regulation occurs automatically regardless of varying operating parameters by the cylinder pressure, which normally urges valve means into a position closing the cylinder, being directly opposed to a force of predetermined size urging said valve means to exhaust the cylinder when cylinder pressure drops below the predetermined size of said force. 

1. In a machine having at least a piston in a cylinder, said piston being moved to do work by the introduction of a compressed fluid into the cylinder on one side of the piston; a selfregulating expansion valve capable of assuming three discrete positions to alternatively connect the cylinder to a compressed fluid supply or to exhaust the cylinder or to close the cylinder so that the cylinder is cut oFf from both supply and exhaust, comprising: relatively movable valve members including ports connectable to exhaust, to the air supply and to the cylinder; resilient means urging the member into a position in which the exhaust port is connected to the cylinder port; means selectively opposing the resilient means to move the members into a position connecting the air supply to the cylinder; and means responsive to the pressure of the expanding air in the cylinder to hold the members in an intermediate position in which the cylinder is closed when the opposition of the means selectively opposing the resilient means is released until the pressure is reduced to a predetermined level.
 2. The machine of claim 1 in which the ports are in the stationary member of the valve.
 3. The machine of claim 2 in which the movable member of the valve is linearly slidable.
 4. The machine of claim 3 in which the resilient means is spring means, or other means providing a nearly constant force.
 5. The machine of claim 4 in which the means selectively opposing the spring means includes a fluid pressure supply means which acts on the movable valve member to overcome spring pressure.
 6. The machine of claim 5 in which the means responsive to the expanding air is a slide-stop which intercepts the slidable member in its return to exhaust position and holds it out of exhaust position and in a position in which the cylinder port is closed.
 7. The machine of claim 6 in which the slide-stop provided with pressure from the cylinder through a duct to the side of the slide-stop opposite that contacting the slidable member.
 8. The machine of claim 7 in which shoulder stop means are provided to limit the movement of the slide-stop to a position which will hold the slidable member in position to close the cylinder port.
 9. A slide type valve system comprising: a fixed member having ports leading respectively; to exhaust, to a cylinder, and to an air pressure source; a slidable member having limited movement relative to the fixed member in opposite end positions of which the cylinder is connected to exhaust and to air pressure, respectively and in an intermediate position of which the cylinder port is closed to close off the cylinder; resilient spring means between the slidable and fixed member urging the slidable member into the position connecting the cylinder to exhaust; air pressure means selectively applicable to the slidable member in opposition to the spring means to move the slidable means to connect the cylinder to the source of air pressure; and a slide stop in axial alignment with the slidable member and movable in response to air pressure in the cylinder into a position to oppose movement of the slidable member into exhaust position and closing the cylinder off from the other ports until the air pressure in the cylinder is reduced to a level which can be overcome by the spring means to return the slidable member to the position connecting the cylinder to exhaust.
 10. The structure of claim 9 in which the slidable member is a spool urged by compression spring means between the spool and the fixed member into the position connecting the air pressure supply to the cylinder.
 11. The structure of claim 10 in which the fixed member is provided with a duct leading from the cylinder to the other side of the slide-stop from that contacting the slidable member, and in which a shoulder for said slide-stop is provided to limit movement of the slide-stop to a position in which it holds the spool in its intermediate position wherein the cylinder port is closed, so that air pressure in the cylinder holds the slide open until the pressure of the air in the cylinder is reduced to the point where the spring overcomes the pressure.
 12. The method of obtaining maximum energy from the expansion of compressed fluid used to do work moving a piston in a cylinder comprising: introducing compressed fluid at a predetermined pressure into the cyLinder; shutting off the cylinder from the compressed fluid supply at a predetermined time and allowing the fluid in the cylinder to move the piston by expansion; pressure in the cylinder providing a force regulated by fluid to oppose opening the cylinder to exhaust the cylinder; and permitting exhaust to occur only when pressure in the cylinder drops below a preselected minimum.
 13. The method of claim 12 in which regulation occurs automatically regardless of varying operating parameters by the cylinder pressure, which normally urges valve means into a position closing the cylinder, being directly opposed to a force of predetermined size urging said valve means to exhaust the cylinder when cylinder pressure drops below the predetermined size of said force. 